The objective of this project is to develop a sensitive screening methodology for te detection of binocular vision problems in young or difficult to test children. This tool will have application in: the evaluation of abnormal vision development, monitoring the effectiveness of interventions for binocular vision problems, and evaluation of binocular vision problems in special populations, such as multiply impaired children. The methodology may also be adapted to the testing of other visual functions in such populations. The specific aims are to refine preferential looking (PL) and operant preferential looking (OPL) methods in children 6 to 60 months old, and to use a combination of these methods to determine the time course of stereoacuity development during this age span. Polarized random-dot stereo targets will be used. This study will: test competing hypotheses with respect to whether stereoacuity changes as a function of age; identify the ages at which significant changes occur; and develp data on expected stereoacuity levels based on age.